Changes in the Emergent Plant Community of Netley-Libau Marsh ...

Vegetation change in Netley-Libau Marsh
Grosshans et al.
METHODS
Aerial Photography and Field
Methods – 2001 Map
Netley-Libau Marsh was photographed as a
mosaic of 106 colour infrared aerial photographs
on 3 August 2001, when plant growth and biomass
were near full development. Photographs were taken
at an altitude of 1,920 m and produced at a scale of
1:10,000. All photography was done with a Wild
RC-30 15/4 UAG-S large format camera. Film used
was Kodak Aerochrome III IR film type 1443.
Infrared film was used to map the distribution of
individual emergent plant species or assemblages
based on their differential reflection of infrared light,
that appears in the final image as shades of red.
Colour photocopies were produced of all aerial
photographs and placed in plastic sheet protectors
for use in the field. Vegetation zones and boundaries
were identified on aerial photographs by ground
verification. Plant species composition, diversity and
density, as well as neighbouring plant communities
were all noted in the field to aid in vegetation zone
classification. An interpretive key of vegetation
infrared signatures for colour infrared aerial
photographs was used from Grosshans (2002),
determined primarily by colour, texture, shadow, and
general appearance (Table 1). This key was used in
conjunction with the aerial photos to identify
vegetation boundaries, and create the digital
vegetation cover map.
Digital Mapping – 2001 Map
All 2001 aerial photographs were scanned at 300
dpi using a Canon flatbed colour copier/scanner. A
Trimble Pathfinder Basic Global Positioning System
(GPS) receiver was used in the field to collect ground
control points (Universal Transverse Mercator Zone
14, North American Datum 83) of visible landmarks
on the aerial photographs to aid in georeferencing
scanned images. Each image was georeferenced using
a combination of the collected GPS data and existing
GPS data from digital orthophotographs based on
aerial photography acquired in August 1991 (Linnet
Geomatics Inc. 2001). Images were geometrically
corrected using ERDAS Imagine 8.5, and used in
ESRI ArcView 3.2 GIS software with the
“IMAGINE image support” extension. A final TIFF
image of the photomosaic was created and seams
between photographs were blended using Adobe
Photoshop 5.5 to produce a large format seamless
color photograph of the entire marsh (Figure 3).
The 2001 digital vegetation map was based on
this mosaic of georeferenced colour infrared
photographs. All digital mapping was done in
ArcView. Vegetation areas were created by on-screen
digitizing in a polygon theme and colour coded with
a unique value-legend type approach based on
vegetation cover type. This approach was adopted
due to sublety in the distinction of discrete plant
assemblages (Table 1). The final vegetation polygon
theme was checked for errors using the “CLU
Quality Control” ArcView extension (Heald 1999).
The map printouts were created in ArcView Layout.
Vegetation Classification – 2001 Map
The vegetation of Netley-Libau Marsh was
categorized into five zones: non-vegetated, emergent
vegetation (permanently-seasonally flooded), wet
meadow (seasonally-temporarily flooded), low
prairie (temporary-no flooding) and upland (no
flooding), distinguished by water depth (surface water
or depth to water-table) and plant community
composition (Appendix 2). Emergent zones are
permanently to seasonally flooded, generally having
standing water throughout most, if not all, of the
growing season. Plant species found in this zone are
also found in waterlogged organic soils above the
water table. Wet meadows are characterized by
flooding for a few weeks in the spring, with 0 to 0.3
m of surface water persisting until mid-summer. Soil
water usually remains within the rooting zone
throughout the growing season. Low prairies
experience temporary to no flooding, with standing
water to saturated soil conditions in the early spring,
with most of the standing water lost rapidly to
seepage and evapotranspiration. Soil moisture within
these areas varies throughout the growing season.
Uplands generally experience little flooding. Surface
water is present only during spring snowmelt and
heavy rains, and is rapidly lost to seepage and
evapotranspiration.
For the 2001 map, these five vegetation zones
were further separated into vegetation classes
represented by one or more dominant species or
distinct species associations. Generally, vegetation
6 DMFS Occasional Publication No. 4